I think a child is my son. How do i go about dna?

I am trying to get help for my son. The mother told him when she was pregnant the baby was possibly his. Then she said no way the baby was his. 15 months later he saw them out and saw the child. He came home and told me he thought the child was his. What does he do now as far as dna? She let him and our family see him. Now she says shes getting married and this guy is gonna adopt him. We love this little boy and in desperate need to know if he is my sons or not. We dont have alot of money but are willing to do all we can.